What Is a Bus? What Kind Does My Mac Have?

Originally published 17 October 2000 as a Mac Daniel article for Low End Mac.

Q: I just finished reading your article about NuBus video cards. Could you further expand on the subject of what a bus is and what type of bus a particular computer has?

A: I guess the best way to imagine it is like a water pipe running from a water source or sources (the expansion slots) to a house (the processor). The bigger the pipe, the more water (or in this analogy, information) it can carry. NuBus, a 32-bit bus running at 10 MHz (think of this as a 10 litre-per-second pump connected to an average-sized pipe), was Apple's standard method of carrying information from the CPU to expansion cards from the introduction of the Mac II in 1987 until the introduction of the PC-standard Peripheral Component Interconnect, or PCI, bus on the motherboards of the 7200, 7500, and 8500 in 1995. The PCI bus is a 32-bit bus running at 33 MHz (same sized pipe as NuBus but with a 33 litre-per-second pump), which means it can carry over three times as much information from the CPU to the expansion slots in the same amount of time. G3-based Macs, for the most part, have a 66 MHz bus, which effectively doubles the rate again, while some also have 64-bit PCI slots running at 33 MHz.

There is also a bus running from the CPU to the memory (RAM), as well as one running to the processor direct (PDS) slot (if present). These days, the motherboard often has all these buses integrated as one "system bus." Except for the iBook, the system bus on current Macs runs at 100 MHz.

Just to be very general, pre-PPC Macs and first generation Power Macs (6100, 7100, and 8100) were NuBus-based, while newer Power Macs are PCI-based. (I know there are exceptions to this, but bear with me here.) For specific information on a particular computer, check out EveryMac's profiles — they have information on what type of expansion capabilities (if any) a particular Macintosh model has.